The present invention relates to a focus error detector for detecting a signal representing a degree of a focus error of an objective lens with respect to an optical disk in an optical recording and reproducing pickup, and more particularly, to a focus error detector which is not sensitive to the tilt or shift of an optical axis.
In general, an optical recording and reproducing pickup is constituted so that in order to place a recording medium such as an optical disk on a focal plane of an objective lens during record and reproduction operations, a signal representing a focus error ratio of an objective lens, caused by the vibration of the optical disk is detected and the objective lens is driven according to the signal.
In detecting the focus error of the objective lens with respect to the optical disk, an astigmatism method disclosed in U.S. Pat. No. 4,841,507 has a drawback in that, even if the optical disk is placed on the focal plane of the objective lens, the detected focus error signal is not zero when the optical axis is tilted or shifted by the vibration or a change of the optical disk over time. Thus, a high density record and reproduction with a small focus intensity is very unstable in operation, due to the frequent generation of servo errors. Also, desired data information is not recorded and reproduced cleanly.
Meanwhile, there is a conventional focus error detecting method, a so-called "beam size" method, which is less sensitive to the tilt or shift of the optical axis by the vibration or optical disk changes over time, as described above. Such a conventional focus error detector is shown in FIG. 1.
Referring to FIG. 1, the light emitted from a light source 1 becomes parallel light by a collimating lens 2, to pass through a beam splitter 3, and is focused onto an optical disk 5 by means of an objective lens 4. The reflected light of optical disk 5 becomes parallel light again by means of objective lens 4, is reflected from beam splitter 3 and then is focused by a focusing lens 6. The focused reflected light is split into two beams by a beam splitter 7. One beam is received from one photo-detector 10 in a photo-detecting unit 9 and the other beam is received from the other photo-detector 11 in photo-detecting unit 9 via a reflective prism 8. Here, ternary detectors are each used for both photo-detectors 10 and 11 as shown in FIG. 2 and are disposed before and after the focal point of focusing lens 6, so that same-sized beam spots are formed in the respective detectors when the optical disk is placed on the focal plane of the objective lens. That is to say, when the optical disk deviates from the focal plane of the objective lens, the size of beam spots formed on both photo-detectors 10 and 11 are changed with respect to each other. At this time, a signal whose magnitude and polarity are changed depending on the focus error is detected by a circuit shown in FIG. 2.
According to the above-described conventional beam size method for detecting focus error, a beam splitter is used for splitting the reflected light into two beams. Also, a reflective prism is used for the purpose of installing two ternary photo-detectors on one plane. Accordingly, costs are increased due to the additional components and miniaturization is difficult due to the resulting large volume.
Also, according to the prior art, since the distance from focusing lens 6 to photo-detector 10 is not the same with that from focusing lens 6 to photo-detector 11, if the reflected light is tilted or shifted due to vibaration or change over time of the optical disk, the spot movement of photo-detector 10 is different from that of photo-detector 11. Thus, even if the optical disk is placed on the focal plane of the objective lens, the tilted or shifted reflected light often results in a focus error.